In general, energy conversion cell devices require several steps to convert energy such as light or a chemical reaction into electrical power. For example, in a solar cell, a first step includes an electronic mechanism whereby a photon excites an electron to a higher, more energetic energy level leaving behind a now vacant energy level or state. Both the photo-excited electrons and the vacant energy state (known as holes) must migrate to separate collection sites (e.g., anode and cathode). In solar cell devices, the separate collection sites for the electrons and holes become the negative and positive contacts. In some cases, the electron and hole travel together as an exciton prior to separation and collection.
The quest for cost effective solar electric generation has fueled research to find inexpensive materials that convert visible light photons into excited charges that can transport the absorbed energy to electrical contacts. Thus far many of the potential inexpensive alternatives to expensive high quality silicon have extremely poor transport. This poor transport inhibits the application of these less-expensive materials in photon energy conversion cells.
In high quality single-crystal silicon solar cells, electrons and holes have large diffusion coefficients and long lifetimes (i.e., the time it takes the photon-excited carrier to recombine with a hole, recombination typically results in heat generation and a failure to contribute collected charge). Single crystal silicon has electron mobility in excess of 100 cm2/V-s. The lifetime of photo-generated minority carriers is sufficient to lead to a diffusion length of less than about 0.5 microns and over 100 microns for amorphous and single crystal silicon, respectively. These diffusion lengths roughly reflect the distance a photo-generated carrier can travel to charge a contact region of a solar cell.
Since amorphous silicon photovoltaic (e.g., solar) cells are typically more than 0.2 microns thick, many photo-generated carries (e.g., electrons, holes, ions, excitons, and radical charge or energy) would recombine prior to collection. The recombination problem can be decreased. However, by using an electric field to assist in carrier collection results in a loss in the available photo-generated power (due to reduced open circuit voltage).
In general, electric field aided collection requires either a graded electrical character (i.e., dopant concentration gradient) and/or a strongly insulating solar cell absorber layer. In the case of amorphous silicon solar cells, a insulating absorber material is used in combination with an assisting electric field. In single crystal silicon solar cells, a dopant gradient is often used near the electrodes to aid carrier collection and reduce recombination.
Recently, the prospect of utilizing low cost solid (e.g., plastic or polymer) and/or liquid junction solar cells has emerged as an important alternative energy prospect. Of particular interest is a dye-sensitized solar cell (a type of liquid junction solar cell) due to its incorporation of ultra low cost materials. Unfortunately, both the solid junction solar cells and the liquid junction solar cells suffer from poor transport of photo-generated charge. For example, the following materials used in solid type solar cells have the following mobility values: oligothiophene based materials have mobilities less than 0.03 cm2/V-s, phthalocyines have mobilities less than 0.01 cm2/V-s, pentacene has a mobility less than 0.62 cm2/V-s, C60 has a mobility value less than 0.08 cm2/V-s, and perylene-diimide has a mobility less of about 1.5×10−5 cm2/V-s. In general, ionic charges diffuse in aqueous solutions with diffusion constants in the 10−5 cm2/s range consistent with a mobility in the 10−4 cm2/V-s range. As a result of these low mobilities, these materials transport photo-generated charge carries poorly, such that a significant portion of available voltage must be used to aid collection thereby reducing the voltage available for external power generation.